1. Field of the Invention
Aspects of the present invention relate to a holographic information storage medium, and an apparatus and method for inspecting a defect thereof.
2. Description of the Related Art
Recently, information storage technology using holography has drawn wide attention. According to a holographic information storage method, information is stored in an inorganic crystal or a photopolymer material sensitive to light in form of an optical interference pattern. The optical interference pattern is formed using two laser beams exhibiting interference. That is, an interference pattern, formed as a reference light and a signal light which have different paths and interfere with each other, is recorded by generating a chemical or physical change on a photosensitive storage medium. In order to reproduce information from the recorded interference pattern, the reference light similar to the recording light is illuminated on the interference pattern recorded on the storage medium. The interference pattern creates a diffraction of the illuminated light so that the signal light is restored and the information is reproduced.
The holographic information storage technology includes a volume holographic method for recording/reproducing information in units of page using volume holography and a micro-holographic method for recording/reproducing information in a single bit using micro-holography. Although the volume holography method has an advantage in processing a large amount of information at the same time, since an optical system needs to be very precisely adjusted, the volume holography method is difficult to commercialize for an information storage apparatus for general consumers.
In the micro-holographic method, a fine interference pattern is formed by allowing two focused lights to interfere with each other at a focal point and a plurality of interference patterns by moving the focal points of the two focused lights on a plane in a storage medium, thus recording information on a recording layer. Furthermore, since the recording layer formed of a photosensitive material such as photopolymer where the interference pattern is recorded has a predetermined thickness and information layers where information is recorded are formed in a depth direction of the recording layer, the information can be recorded in three dimensions. As the information is recorded in a plurality of the information layers in the recording layer, an area where a light beam passing through an objective lens meets a surface of the storage medium, that is, a spot area, varies according to the information layer where recording is performed. When the number of the information layers is 4, 8, or 16, a difference between the minimum area and the maximum area where the light beam meets the surface of the storage medium increases. Accordingly, an effect by dust having the same size on the surface of the different storage medium varies according to the number of information layers and a degree of the variation increases as the number of the information layers increases. Thus, a method to remove the effect (error) created by the dust for each information layer of the storage medium is needed.
FIG. 1 illustrates a storage medium used in a micro-holographic method in which information is recorded in multiple layers of such storage medium. Referring to FIG. 1, a reference light L1 and a signal light L2, respectively, pass through first and second objective lenses OL1 and OL2 and form a focal point F on a recording layer 12 of a holographic information storage medium 10. Information is recorded in the form of a hologram 13 due to the interference between the reference light L1 and the signal light L2 at a position where the focal point F is formed. The information recorded by the hologram 13 forms an information layer IL at the same depth from the surface of a storage medium 10. As the depth of the focal point F of the reference light L1 and the signal light L2 varies, multiple information layers IL are formed at different depths from the surface of the storage medium 10. The distance from the surfaces of the first and second protective layers 11 and 19, covering both sides of the recording layer 12, to each information layer IL is referred to as the cover layer thickness. In the present example, since the storage medium 10 is a dual side incident type, the cover layer thickness can be defined based on a surface on which the reference light L1 is incident or a surface on which the signal light L2 is incident.
FIG. 2 illustrates spot areas in each information layer inside the recording layer according to the different thicknesses of the cover layer. In FIG. 2, the cover layer thickness is defined based on the surface on which the reference light L1 is incident. Referring to FIG. 2, the cover layer thickness is different for each information layer and accordingly the spot area that is an area where the reference light L1 meets the surface of the first protective layer 11 is different for each information layer IL. For example, when a first cover layer thickness T1 for a first information layer IL1 is smaller than a second cover layer thickness T2 for a second information layer IL2, a first spot area S1 formed by the reference light L1 having a focal point formed on the first information layer IL1 is smaller than a second spot area S2 formed by the reference light L1 having a focal point formed on the second information layer IL2.
When an incident/reflected beam is blocked by defects such as dust, fingerprints, scratches, or bubbles, since the spot area formed on the surface of the storage medium varies for each information layer, the amount of the light reflected by a defect generated on the surface of the storage medium varies for each information layer. That is, the amount of reflected light is proportional to the difference between the spot area and the area where incident/reflected light is blocked by a defect. This is because the spot area varies according to the cover layer thickness for each information layer while “the area where incident/reflected light is blocked by a defect” is constant for each information layer with respect to the same defect on the surface of the storage medium. In general, when the amount of reflected light is less than a certain level, a detection system detects an error. Accordingly, for a defect of the same size, an error is detected in a certain layer and an error is not detected in another layer. Thus, in spite of the fact that each information layer is affected differently by the same defect, a method for inspecting a defect in a holographic information storage medium inspects the overall area of the recording layer and determines whether there is a defect. As described above, since the overall area of the recording layer is inspected, a lot of time is spent inspecting for a defect. Also, even in the middle of recording, a defect inspection operation is performed, thus consuming much time.